Drill bit for downhole electrocrushing drilling

ABSTRACT

An electrocrushing drill bit, comprising a bit body, an electrode coupled to a power source and the bit body, the electrode having a distal portion for engaging with a surface of a wellbore; a ground ring coupled to the bit body proximate to the electrode and having a distal portion for engaging with the surface of the wellbore, the electrode and the ground ring positioned in relation to each other such that an electric field produced by a voltage applied between the ground ring and the electrode is enhanced at a portion of the electrode proximate to the distal portion of the electrode and at a portion of the ground ring proximate to the distal portion of the ground ring. wherein the ground ring further includes a fluid flow port; and an insulator coupled to the bit body between the electrode and the ground ring.

TECHNICAL FIELD

The present disclosure relates generally to downhole electrocrushingdrilling and, more particularly, to drill bits used in downholeelectrocrushing drilling.

BACKGROUND

Electrocrushing drilling uses pulsed power technology to drill aborehole in a rock formation. Pulsed power technology repeatedly appliesa high electric potential across the electrodes of an electrocrushingdrill bit, which ultimately causes the surrounding rock to fracture. Thefractured rock is carried away from the bit by drilling fluid and thebit advances downhole.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an elevation view of an exemplary downhole electrocrushingdrilling system used in a wellbore environment;

FIG. 2 is a perspective view of exemplary components of a bottom holeassembly for a downhole electrocrushing drilling system;

FIG. 3A is a perspective view of an exemplary electrode for a downholeelectrocrushing drill bit;

FIG. 3B is a cross-sectional view of the electrode shown in FIG. 3A;

FIG. 4A is a perspective view of an exemplary electrode for a downholeelectrocrushing drill bit;

FIG. 4B is a cross-sectional view of the electrode shown in FIG. 4A;

FIG. 5A is a perspective view of an exemplary electrode for a downholeelectrocrushing drill bit;

FIG. 5B is a cross-sectional view of the electrode shown in FIG. 5A;

FIG. 5C is a cross-sectional view of an alternative design of theelectrode shown in FIG. 5A;

FIG. 6A is a perspective view of an exemplary ground ring for a downholeelectrocrushing drill bit;

FIG. 6B is a cross-sectional view of the ground ring shown in FIG. 6A;

FIG. 7 is a perspective view of an electrocrushing drill bit includingmultiple electrodes and a ground ring;

FIG. 8 is a perspective view of an electrocrushing drill bit includingmultiple electrodes arranged in multiple rows with an external groundring and an intermediate ground ring;

FIG. 9 is a perspective view of an electrocrushing drill bit includingmultiple electrodes, an outer ground ring, and an intermediate groundring traversing the outer ground ring to divide the bit into threeregions;

FIG. 10 is a perspective view of an electrocrushing drill bit includingmultiple electrodes, an outer ground ring, and an intermediate groundring traversing the outer ground ring to divide the electrocrushingdrill bit into nine regions;

FIG. 11 is a perspective view of an electrocrushing drill bit includingmultiple electrodes located within openings in a ground ring structure;

FIG. 12 is a perspective view of an electrocrushing drill bit includingmultiple electrodes arranged in rows, a central electrode, and a groundring; and

FIG. 13 is a flow chart of exemplary method for drilling a wellbore.

DETAILED DESCRIPTION

Electrocrushing drilling may be used to form wellbores in subterraneanrock formations for recovering hydrocarbons, such as oil and gas, fromthese formations. Electrocrushing drilling uses pulsed-power technologyto repeatedly fracture the rock formation by repeatedly deliveringhigh-energy electrical pulses to the rock formation. A drill bit usedfor electrocrushing drilling includes an electrode and a ground ringcoupled to a power source. The electrode and ground ring have contoursdesigned to enhance, concentrate, or otherwise manage the electric fieldsurrounding the drill bit. The electrode and ground ring also have fluidflow ports and openings to facilitate the flow of electrocrushingdrilling fluid into and out of the drilling field. During a drillingoperation, the electric field surrounding the drill bit is such that anarc forms and spans the electrode and the ground ring and penetrates therock formation. The electrocrushing drilling fluid insulates thecomponents of the drill bit and removes rock cuttings from the drillingfield. As such, an electrocrushing drill bit designed according to thepresent disclosure may provide for more efficient drilling and removalof cuttings during the drilling operation.

There are numerous ways in which electrocrushing drill bits may beimplemented in a downhole electrocrushing pulsed-power system. Thus,embodiments of the present disclosure and its advantages are bestunderstood by referring to FIGS. 1 through 7 , where like numbers areused to indicate like and corresponding parts.

FIG. 1 is an elevation view of an exemplary electrocrushing drillingsystem used to form a wellbore in a subterranean formation. AlthoughFIG. 1 shows land-based equipment, downhole tools incorporatingteachings of the present disclosure may be satisfactorily used withequipment located on offshore platforms, drill ships, semi-submersibles,and drilling barges (not expressly shown). Additionally, while wellbore116 is shown as being a generally vertical wellbore, wellbore 116 may beany orientation including generally horizontal, multilateral, ordirectional.

Drilling system 100 includes drilling platform 102 that supports derrick104 having traveling block 106 for raising and lowering drill string108. Drilling system 100 also includes pump 124, which circulateselectrocrushing drilling fluid 122 through a feed pipe to drill string110, which in turn conveys electrocrushing drilling fluid 122 downholethrough interior channels of drill string 108 and through one or moreorifices in electrocrushing drill bit 114. Electrocrushing drillingfluid 122 then circulates back to the surface via annulus 126 formedbetween drill string 108 and the sidewalls of wellbore 116. Fracturedportions of the formation are carried to the surface by electrocrushingdrilling fluid 122 to remove those fractured portions from wellbore 116.

Electrocrushing drill bit 114 is attached to the distal end of drillstring 108. In some embodiments, power to electrocrushing drill bit 114may be supplied from the surface. For example, generator 140 maygenerate electrical power and provide that power to power-conditioningunit 142. Power-conditioning unit 142 may then transmit electricalenergy downhole via surface cable 143 and a sub-surface cable (notexpressly shown in FIG. 1 ) contained within drill string 108 orattached to the side of drill string 108. A pulse-generating circuitwithin bottom-hole assembly (BHA) 128 may receive the electrical energyfrom power-conditioning unit 142, and may generate high-energy pulses todrive electrocrushing drill bit 114.

The pulse-generating circuit within BHA 128 may be utilized torepeatedly apply a high electric potential, for example up to orexceeding 150 kV, across the electrodes of electrocrushing drill bit114. Each application of electric potential may be referred to as apulse. When the electric potential across the electrodes ofelectrocrushing drill bit 114 is increased enough during a pulse togenerate a sufficiently high electric field, an electrical arc formsthrough a rock formation at the bottom of wellbore 116. The arctemporarily forms an electrical coupling between the electrodes ofelectrocrushing drill bit 114, allowing electric current to flow throughthe arc inside a portion of the rock formation at the bottom of wellbore116. The arc greatly increases the temperature and pressure of theportion of the rock formation through which the arc flows and thesurrounding formation and materials. The temperature and pressure aresufficiently high to break the rock itself into small bits or cuttings.This fractured rock is removed, typically by electrocrushing drillingfluid 122, which moves the fractured rock away from the electrodes anduphole.

As electrocrushing drill bit 114 repeatedly fractures the rock formationand electrocrushing drilling fluid 122 moves the fractured rock uphole,wellbore 116, which penetrates various subterranean rock formations 118,is created. Wellbore 116 may be any hole drilled into a subterraneanformation or series of subterranean formations for the purpose ofexploration or extraction of natural resources such as, for example,hydrocarbons, or for the purpose of injection of fluids such as, forexample, water, wastewater, brine, or water mixed with other fluids.Additionally, wellbore 116 may be any hole drilled into a subterraneanformation or series of subterranean formations for the purpose ofgeothermal power generation.

Although drilling system 100 is described herein as utilizingelectrocrushing drill bit 114, drilling system 100 may also utilize anelectrohydraulic drill bit. An electrohydraulic drill bit may have oneor more electrodes and ground ring similar to electrocrushing drill bit114. But, rather than generating an arc within the rock, anelectrohydraulic drill bit applies a large electrical potential acrossthe one or more electrodes and ground ring to form an arc across thedrilling fluid proximate the bottom of wellbore 116. The hightemperature of the arc vaporizes the portion of the fluid immediatelysurrounding the arc, which in turn generates a high-energy shock wave inthe remaining fluid. The one or more electrodes of electrohydraulicdrill bit may be oriented such that the shock wave generated by the arcis transmitted toward the bottom of wellbore 116. When the shock wavehits and bounces off of the rock at the bottom of wellbore 116, the rockfractures. Accordingly, drilling system 100 may utilize pulsed-powertechnology with an electrohydraulic drill bit to drill wellbore 116 insubterranean formation 118 in a similar manner as with electrocrushingdrill bit 114.

FIG. 2 is a perspective view of exemplary components of the bottom holeassembly for downhole electrocrushing drilling system 100. Bottom-holeassembly (BHA) 128 may include pulsed-power tool 230. BHA 128 may alsoinclude electrocrushing drill bit 114. For the purposes of the presentdisclosure, electrocrushing drill bit 114 may be integrated within BHA128, or may be a separate component that is coupled to BHA 128.

Pulsed-power tool 230 may be coupled to provide pulsed electrical energyto electrocrushing drill bit 114. Pulsed-power tool 230 receiveselectrical power from a power source via cable 220. For example,pulsed-power tool 230 may receive electrical power via cable 220 from apower source on the surface as described above with reference to FIG. 1, or from a power source located downhole such as a generator powered bya mud turbine. Pulsed-power tool 230 may also receive electrical powervia a combination of a power source on the surface and a power sourcelocated downhole. Pulsed-power tool 230 converts the electrical powerreceived from the power source into high-energy electrical pulses thatare applied across electrode 208 and ground ring 250 of electrocrushingdrill bit 114.

Referring to FIG. 1 and FIG. 2 , electrocrushing drilling fluid 122 mayexit drill string 108 via opening 209 surrounding electrode 208. Theflow of electrocrushing drill fluid 122 out of opening 209 allowselectrode 208 to be insulated by the electrocrushing drilling fluid.While one electrode 208 is shown in FIG. 2 , electrocrushing drill bit114 may include multiple electrodes 208. Electrocrushing drill bit 114may include solid insulator 210 surrounding electrode 208 and one ormore orifices (not expressly shown in FIG. 1 or 2 ) on the face ofelectrocrushing drill bit 114 through which electrocrushing drillingfluid 122 exits drill string 108. Such orifices may be simple holes, orthey may be nozzles or other shaped features. Because fines are nottypically generated during electrocrushing drilling, as opposed tomechanical drilling, electrocrushing drilling fluid 122 may not need toexit the drill bit at as high a pressure as the drilling fluid inmechanical drilling. As a result, nozzles and other features used toincrease drilling fluid pressure may not be needed. However, nozzles orother features to increase electrocrushing drilling fluid 122 pressureor to direct electrocrushing drilling fluid may be included for someuses. Additionally, the shape of solid insulator 210 may be selected toenhance the flow of electrocrushing drilling fluid 122 around thecomponents of electrocrushing drill bit 114.

Electrocrushing drilling fluid 122 is typically circulated throughdrilling system 100 at a flow rate sufficient to remove fractured rockfrom the vicinity of electrocrushing drill bit 114. In addition,electrocrushing drilling fluid 122 may be under sufficient pressure at alocation in wellbore 116, particularly a location near a hydrocarbon,gas, water, or other deposit, to prevent a blowout.

Electrocrushing drill bit 114 may include bit body 255, electrode 208,ground ring 250, and solid insulator 210. Electrode 208 may be placedapproximately in the center of electrocrushing drill bit 114. Thedistance between electrode 208 and ground ring 250 may be a minimum ofapproximately 0.4 inches and a maximum of approximately 4 inches. Thedistance between electrode 208 and ground ring 250 may be based on theparameters of the electrocrushing drilling operation. For example, ifthe distance between electrode 208 and ground ring 250 is too small,electrocrushing drilling fluid 122 may break down and the arc betweenelectrode 208 and ground ring 250 may not pass through the rock.However, if the distance between electrode 208 and ground ring 250 istoo large, electrocrushing drilling bit 114 may not have adequatevoltage to form an arc through the rock. For example, the distancebetween electrode 208 and ground ring 250 may be at least 0.4 inches, atleast 1 inch, at least 1.5 inches, or at least 2 inches. The distancebetween electrode 208 and ground ring 250 may be based on the diameterof electrocrushing drill bit 114. The distance between electrode 208 andground ring 250 may be generally symmetrical or may be asymmetrical suchthat the electric field surrounding the electrocrushing drill bit has asymmetrical or asymmetrical shape. The distance between electrode 208and ground ring 250 allows electrocrushing drilling fluid 122 to flowbetween electrode 208 and ground ring 250 to remove vaporization bubblesfrom the drilling area. If drilling system 100 experiences vaporizationbubbles in electrocrushing drilling fluid 122 near electrocrushing drillbit 114, the vaporization bubbles may have deleterious effects. Forinstance, vaporization bubbles near electrode 208 may impede formationof the arc in the rock. Electrocrushing drilling fluid 122 may becirculated at a flow rate also sufficient to remove vaporization bubblesfrom the vicinity of electrocrushing drill bit 114.

Electrode 208 has three sections: face 216, body 217, and stem 218. Face216 is a distal portion of electrode 208 in contact with the rock duringan electrocrushing drilling operation. For example, face 216 may engagewith a portion of the wellbore, such as wellbore 116 shown in FIG. 1 .Body 217 couples face 216 to stem 218. Stem 218 couples electrode 208 toelectrocrushing drill bit 114. Electrode 208 may have any suitablediameter based on the drilling operation. For example, electrode 208 mayhave a diameter between approximately two and approximately ten inches.In some embodiments electrode 208 may be smaller than two inches indiameter. The diameter of the electrode may be based on the diameter ofelectrocrushing drill bit 114 and the distance between electrode 208 andground ring 250, as described above.

The geometry of electrode 208 affects the electric field surroundingelectrocrushing drill bit 114 during electrocrushing drilling. Forexample, the geometry of electrode 208 may be designed to result in anenhanced electric field surrounding electrode 208 so that the arcsinitiate at electrode 208 and terminate on ground ring 250, or viceversa such that the arc initiates from ground ring 250 and terminate onelectrode 208. The electric field surrounding electrode 208 may bedesigned so that most of the arcs initiating between electrode 208 andground ring 250 do so through a path or multitude of paths that resultsin more efficient rock removal, for example a path or paths through therock. Similarly, the electric field surrounding electrode 208 may bedesigned so as to minimize the arcs initiating between electrode 208 andground ring 250 that do so through a path or multitude of paths thatresults in less efficient rock removal, for example path or pathsshort-cutting through the drilling fluid without penetrating the rock.For example, face 216 of electrode 208 may be engaged with a surface ofthe wellbore and a distal portion of ground ring 250 may also be engagedwith the surface of the wellbore. The electric field may be designedsuch that the electric field is enhanced at a portion of electrode 208proximate to face 216 and on a portion of ground ring 250 proximate tothe distal portion of ground ring 250. An enhanced electric field in aregion surrounding electrocrushing drill bit 114 may result in anincreased electric flux in that region. For example, the electric fieldE_(s) in the vicinity of a specifically shaped conducting structure willbe larger than the average macroscopic electrical field created by theapplied voltage over the average spacing E_(applied) by the fieldenhancement factor, y, defined by the equation below:

$\gamma = \frac{E_{s}}{E_{applied}}$

The geometry of electrode 208 includes the profile of face 216, theshape of body 217, and contours of transitions between face 216, body217, and stem 218. For example, face 216 may have a flat profile, aconcave profile, or a convex profile. The profile may be based on thedesign of the electric field surrounding the electrocrushing drill bit.Body 217 may be generally conical shaped, cylindrical shaped,rectangular shaped, polyhedral shaped, tear drop shaped, rod shaped, orany other suitable shape. The transitions between face 216 and body 217may be contoured to result in electric field conditions that are eitherfavorable or unfavorable for arc initiation or termination. For example,the transition between face 216 and body 217 may have a sharp radius ofcurvature such that the electric field conditions are favorable for anarc to initiate and/or terminate at the transition between face 216 andbody 217. In contrast, the transition between body 217 and stem 218 mayhave a gentle radius of curvature such that the conditions are notfavorable for arc initiation and/or termination at the transitionbetween body 217 and stem 218. A radius of curvature of a transition isthe radius of a circle of which the arc of the transition is a part. Byway of example, a sharp radius of curvature may be a radius greater than0.01 inches, and sometimes in the range of approximately 0.05 toapproximately 0.15 inches, such as approximately 0.094 inches, and agentle radius of curvature may be a radius in the range of approximately0.15 to approximately 1.0 inches, such as approximately 0.25 inches,approximately 0.5 inches, approximately 0.75 inches, or approximately1.0 inches. The ratio of the gentle radius of curvature to the sharpradius of curvature may be by approximately 2:1 or more, and may be upto 5:1, 10:1, or substantially greater than 10:1. The gentle radius maybe determined based on the geometry of the surrounding structures onelectrocrushing drill bit 114 and the shape of the electric field for agiven electrocrushing drilling operation. For example, the electricfields on electrode 208 may be a function of the geometry of ground ring250 and the geometry and material of insulator 210. For example, theradius of the edge of electrode 208 and the shape of electrode 208 mayaffect the interaction of electrocrushing drill bit 114 with the rock.Additionally, the structure of ground ring 250 may be adjusted to changethe electric field distribution on electrode 208. Further, the materialused to form insulator 210 and the configuration of insulator 210 may beadjusted to change the electric field on electrode 208. In someexamples, the dielectric constant of the electrocrushing drilling fluidand the geometry of the rock fragments and the wellbore during thedrilling process may affect the instantaneous electric fielddistribution on electrode 208. The transitions are shown in more detailin FIGS. 3A-5B. Electrode 208 may be any of the electrodes shown inFIGS. 3A-5B.

The geometry of electrocrushing drill bit 114, and specifically certaindimensions between electrode 208 and ground ring 250, may be designed tomaximize the occurrence of arc paths between the electrode and groundring which travel through the rock, and/or to minimize short-cut pathsfor arcs to travel between the electrode and ground ring. Body 217, orbody 217 in combination with stem 218, may be shaped to result in afirst minimum distance between electrode 208 and ground ring 250, with asubstantial portion of the electrode's conductive surface in the axialdirection, perpendicular to face 216, being at a greater distance fromground ring 250. The first minimum distance may be a distance less thanthe average distance between electrode 208 and ground ring 250. Thefirst minimum distance may result in a relative enhancement orconcentration of the electric field at the perimeter of face 216 versusthe balance of the axial extent of electrode 208, for example such thatfirst minimum distance is at least approximately 15% less than theaverage distance between electrode 208 and ground ring 250, at leastapproximately 25% less than the average distance between electrode 208and ground ring 250, or at least approximately 50% less than the averagedistance between electrode 208 and ground ring 250. A conical shapedground ring as shown in FIG. 2 may achieve this criterion, as may asemi-sphere or certain other geometries. For example, in FIG. 2 , thefirst minimum distance may be the distance between the perimeter of face216 and ground ring 250 while the average distance between electrode 308and ground ring 250 is calculated including the distance between body217 and ground ring 250 and stem 218 and ground ring 250. The firstminimum distance may be such that the electric field is enhanced orconcentrated on a portion of electrode 208 proximate to face 216 and ona portion of ground ring 250 proximate to the distal portion of groundring 250.

Ground ring 250 may function as an electrode and provide a location onthe electrocrushing drill bit where an arc may initiate and/orterminate. Ground ring 250 also provides one or more fluid flow ports260 such that electrocrushing drilling fluids flow through fluid flowports 260 carry fractured rock and vaporization bubbles away from thedrilling area. Further, ground ring 250 provides structural support forelectrocrushing drill bit 114 to support the downforce caused by theweight of the electrocrushing drilling components uphole fromelectrocrushing drill bit 114, such as drill string 108 shown in FIG. 1. Electrocrushing drill bit 114 may additionally include an additionalstructural component (not expressly shown) that supports the downforcecreated by the weight of the electrocrushing drilling components upholefrom electrocrushing drill bit 114. For example, an insulative ring orstuds may be located on electrocrushing drill bit 114 to bear some orall of the weight of the electrocrushing drilling components and theweight of some or all of the drill string. As another example, astructural support structure, physically separated from but coupled tothe ground ring electrode, may be used to support the weight ofelectrocrushing drilling components and drill string.

FIG. 3A is a perspective view of an exemplary electrode for a downholeelectrocrushing drill bit. FIG. 3B is a cross-sectional view of theelectrode shown in FIG. 3A. Electrode 308 provides a similar functionand has similar features as electrode 208 shown in FIG. 2 .

High electrical energy pulses from a power source may be applied toelectrode 308 to generate an arc as described in more detail in FIGS. 1and 2 . As described with reference to FIG. 2 , the contours of thetransitions between parts of electrode 308 affect the electric fieldsurrounding the electrocrushing drill bit. For example, the transitionbetween face 316 and body 317, edge 312, may have a sharp radius ofcurvature, as described above with reference to FIG. 2 , such that theelectric field conditions are favorable for an arc to initiate and/orterminate at edge 312. In contrast, transition 314, between body 317 andstem 318, may have a gentle radius of curvature such that the electricfield conditions are not favorable for arc initiation and/ortermination.

Electrode 308 may further include fluid flow opening 309 extendingthrough stem 318 and body 317 to face 316 to direct electrocrushingdrilling fluids from a drill string, such as drill string 108 shown inFIG. 1 , downhole to the electrocrushing drilling bit. For example, theelectrocrushing drill bit may be coupled to the drill string andelectrocrushing drilling fluid may flow downhole through the drillsting, to electrocrushing drill bit and exit through fluid flow opening309. A portion or all of the fluid flowing through the drill string mayexit through fluid flow opening 309. Fluid flow opening 309 may becentered on face 316, as shown in FIGS. 3A and 3B, or may be offsetradially. The flow path may be coaxial with electrode 308 or may be atan angle offset from the centerline of electrode 308. Fluid flow opening309 may have a cross sectional area designed to result in higher fluidvelocity than the flow through the drill string, and may include anorifice or jet.

Alternatively, fluid flow opening 309 may be used to accept a bolt toattach electrode 308 to the internal structure of the BHA (not expresslyshown) to which electrode 308 is attached. Electrode 308 may furtherinclude slots 319 that facilitate the flow of electrocrushing drillingfluids around electrode 308. The presence of slots 319 may modify thedirection and/or velocity of the flow of electrocrushing drilling fluidthrough the drilling area. Some slots 319 may be channels on face 316 ofelectrode 308, as shown by slot 319 a in FIG. 3B, that extends partiallythrough body 317. Other slots 319 may extend through body 317, as shownby slot 319 b in FIG. 3B. Some or all slots 319 may terminate short ofintersecting with fluid flow opening 309, as shown in FIGS. 3A and 3Band some or all slots 319 may intersect with fluid flow opening 309.Electrode 308 may have any combination of slots 319. As shown in FIG.3A, edge 320 of each slot 319 may have a sharp radius of curvature, asdescribed above with reference to FIG. 2 , to create favorableconditions in the electric field for arc initiation and/or termination.Edge 320 of each slot 319 may also have a sharp radius or any otherradius of curvature suitable for the drilling and/or fabricationprocess.

Electrode 308 may be manufactured from any material that can withstandthe conditions in a wellbore and has sufficient conductivity to conductthousands of amps per pulse without structurally damaging the electrode,such as steel in the 41 family (often designated as the 41xx family, forexample 4140 steel), carbon alloyed steel, stainless steel, nickel andnickel alloys, copper and copper alloys, titanium and titanium alloys,chromium and chromium alloys, molybdenum and molybdenum alloys, dopedceramics, composite materials using a matrix material having a highmelting point, such as tungsten and a reinforcement material having ahigh conductivity and low melting point, such as copper, brass, silver,or gold, and combinations thereof. The conductivity of electrode 308 maybe a function of the geometry of electrode 308 and the shape of the arcthat forms between electrode 308 and the ground ring or other electrodeson the electrocrushing drilling bit. For example, the minimumconductivity of electrode 308 may be based on the voltage requirementsof the electrocrushing drilling operation and such conductivities(measured at 20° C.) may be at least approximately 0.5×10{circumflexover ( )}6 1/ohm-meter, at least approximately 1.0×10{circumflex over( )}7 1/ohm-meter, or higher. When an arc initiates or terminates atelectrode 308, the temperature at the initiation or termination pointincreases such that the temperature melts the surface of electrode 308.Arc creation is often accompanied by a shock wave. When the shock waveimpacts the melted surface of electrode 308, a portion of the meltedsurface may separate from the remainder of electrode 308 and be carrieduphole with the electrocrushing drilling fluid. Therefore, to preventmaterial loss, the areas of electrode 308, for example edges 312 and/or320, having electric field conditions favorable to arc initiation and/ortermination may be coated with or made of a metal matrix composite. Themetal matrix composite may be formed of a matrix material having a highmelting point, and/or high resistance to electrical erosion, such astungsten, carbide, ceramic, polycrystalline diamond compact, carbonfiber, graphene, graphite, olivene (FEPO₄), carbon tubes or combinationsthereof, infused with a metal having a low melting point, such ascopper, gold, silver, indium, or combinations thereof. For example, themetal matrix composite may be a tungsten and copper composite such asELKONITE®, manufactured and sold by CMW Inc. of Indianapolis, Ind. Themelting point of the matrix material may be higher than the meltingpoint of the infused metal. During arc initiation and/or termination,the infused metal may melt while the matrix material remains solid tohold the melted infused metal in place during the shock wave motion.After the temperature decreases, the infused metal solidifies withoutany material loss.

Although FIGS. 3A-3B illustrate a particular electrode design having acertain combination of features, electrode 308 may use any suitablecombination of features to generate an arc. Such features may includeany one or more of the features of electrode 408 shown in FIGS. 4A-4Band/or electrode 508 shown in FIGS. 5A-5B, such as one or more notchesand/or a spring.

FIG. 4A is a perspective view of an exemplary electrode for a downholeelectrocrushing drill bit. FIG. 4B is a cross-sectional view of theelectrode shown in FIG. 4A. Electrode 408 provides a similar functionand has similar features as electrode 208 shown in FIG. 2 .

As described with respect to FIG. 2 , the contours of the transitionsbetween parts of electrode 308 affect the electric field surrounding theelectrocrushing drill bit. For example, edge 412 may have a sharp radiusof curvature such that the electric field conditions are favorable forarc initiation and/or termination at edge 412. In contrast, transition414 may have a gentle radius of curvature such that the electric fieldconditions are not favorable for arc initiation and/or termination.

Electrode 408 may further include one or more notches 422 along edge412. The presence of notches 422 may change the electric fieldsurrounding electrode 408 by increasing the electric field nearelectrode 408. Edge 412 of notches 422 may have a sharp radius ofcurvature to create conditions favorable for arc initiation and/ortermination by providing a larger perimeter of electrode 408 having asharp radius of curvature than the perimeter of a smooth edge (as shownin FIG. 3A). While notches 422 are shown as U-shaped in FIG. 4A, notches422 may have any suitable shape including triangular, rectangular,polygonal, circular, or any combination thereof. While notches 422 areshown as indentations in edge 412, in some examples edge 412 may havediscontinuities that protrude out from edge 412. Additionally, whileelectrode 408 is shown as including notches 422, any discontinuity alongedge 412 may achieve a similar effect as notches 422. For example, edge412 may be serrated or dimpled. Additionally, discontinuities on face416 may also achieve a similar effect as discontinuities along edge 412.For example, face 416 may include buttons, dimples, or protrusions. Thesize of the discontinuities along edge 412 may be a function of thespacing between electrode 408 and a ground ring, the radius of electrode408, the type of rock being drilled, the fluid flow path of theelectrocrushing drilling fluid, or any combination thereof. Thediscontinuities may protrude outward, or indent inward, from edge 412 orface 416, a distance (measured perpendicular to edge 412 or face 416)from approximately 0.03 inch to approximately 0.12 inch, or up toapproximately 0.25 inch or more. The aggregate perimeter length ofdiscontinuities along edge 412 (i.e., the portion of the perimeterinterrupted by such discontinuities) may total approximately 5% toapproximately 30% of the perimeter length, approximately 25% toapproximately 75% of the perimeter length, or more. The aggregate areaof discontinuities on face 416 (i.e., the portion of the face surfacearea interrupted by such discontinuities) may total approximately 5% toapproximately 30% of the surface area of face 416, approximately 25% toapproximately 75% of the surface area, or more. The discontinuities maybe distributed uniformly about the perimeter of edge 412 or uniformlyupon face 416, or may be enhanced or concentrated in portions of theperimeter of edge 412 (e.g., enhanced or concentrated in center of eachof 4 quadrants) or portions of the area of face 416 (e.g., enhanced orconcentrated in a band on face 416 near edge 412, or in multipleconcentric bands, or enhanced or concentrated in other zones within face416).

Electrode 408 may be manufactured from materials similar to thematerials described with respect to electrode 308 in FIGS. 3A-3B, suchas steel in the 41 family (often designated as the 41xx family, forexample 4140 steel), carbon alloyed steel, stainless steel, nickel andnickel alloys, copper and copper alloys, titanium and titanium alloys,chromium and chromium alloys, molybdenum and molybdenum alloys, dopedceramics, and combinations thereof. Additionally, the areas of electrode408 having electric field conditions favorable to arc initiation and/orformation may be coated with or made of a metal matrix composite asdescribed in FIGS. 3A-3B.

Although FIGS. 4A-4B illustrate a particular electrode design having acertain combination of features, electrode 408 may use any suitablecombination of features to generate an arc. Such features may includeany one or more of the features of electrode 308 shown in FIGS. 3A-3Band/or electrode 508 shown in FIGS. 5A-5B, such as a fluid flow port,one or more slots, and/or a spring.

FIG. 5A is a perspective view of an exemplary electrode for a downholeelectrocrushing drill bit. FIG. 5B is a cross-sectional view of theelectrode shown in FIG. 5A. Electrode 508 provides a similar functionand has similar features as electrode 208 shown in FIG. 2 .

As described with respect to FIG. 2 , the contours of the transitionsbetween parts of electrode 208 affect the electric field surrounding theelectrocrushing drill bit. For example, edge 512 may have a sharp radiusof curvature such that the electric field conditions at edge 512 arefavorable for arc initiation and/or termination. In contrast, transition514, where body 517 joins stem 518 of electrode 508, may have a gentleradius of curvature such that the electric field conditions are notfavorable for arc initiation and/or termination.

Similar to electrode 408 shown in FIGS. 4A-4B, electrode 508 may furtherinclude one or more notches 522 along edge 512. The presence of notches522 may change the electric field surrounding electrode 508 byincreasing the electric field near electrode 508. Edge 512 of notches522 may have a sharp radius of curvature to create conditions favorablefor arc initiation and/or termination by providing a larger perimeter ofelectrode 508 having a sharp radius of curvature than the perimeter of asmooth edge (as shown on electrode 308 in FIG. 3A). While notches 522are shown as U-shaped in FIG. 5A, notches 522 may have any suitableshape including triangular, rectangular, polygonal, circular, or anycombination thereof.

Electrode 508 may be manufactured from materials similar to thematerials described with respect to electrode 308 in FIGS. 3A-3B, suchas steel in the 41 family (often designated as the 41xx family, forexample 4140 steel), carbon alloyed steel, stainless steel, nickel andnickel alloys, copper and copper alloys, titanium and titanium alloys,chromium and chromium alloys, molybdenum and molybdenum alloys, dopedceramics, and combinations thereof. Additionally, the areas of electrode508 having electric field conditions favorable to arc initiation and/orformation may be coated with or made of a metal matrix composite asexplained in FIGS. 3A-3B.

Electrode 508 may additionally include one or more slots 519 thatfacilitate the flow of electrocrushing drilling fluid around electrode508. Some slots 519 may be channels on face 516 of electrode 508, asshown by slot 519 a in FIG. 5B, that extend partially through body 517.Other slots 519 may extend through body 517, as shown by slot 519 b inFIG. 5B. Electrode 508 may have any combination of slots 519. Edge 520of each slot 519 may have a sharp radius of curvature to createfavorable conditions in the electric field for arc initiation and/ortermination.

Electrode 508 may further include a biasing device that urges electrode508 away from the drill string and into contact with the rock throughwhich the electrocrushing drill bit is drilling. For example, as shownin FIG. 5 , electrode 508 includes internal spring 524. Spring 524 maybe located in a fluid flow port, such as fluid flow port 309 shown inFIG. 3B, or a bolt attachment socket as described with reference toFIGS. 3A-3B. The action of spring 524 may then move electrode 508 in adirection away from the drill string and toward the rock such that face516 maintains contact with the rock during the electrocrushing drillingoperation. In some electrocrushing drill bits, spring 524 may bereplaced with piston 525 (as shown in FIG. 5C) and/or a magnetic devicethat cause face 516 to maintain contact with the rock. Piston 525 may beactivated by the pressure of the electrocrushing drilling fluid in thedrill string. The magnetic device may be activated using the currentpulses sent to electrode 508.

Although FIGS. 5A-5C illustrate a particular electrode design having acertain combination of features, electrode 508 may use any suitablecombination of features to generate an arc. Such features may includeany one or more of the features of electrode 308 or electrode 408 shownin FIGS. 3A-4B, such as a fluid flow port.

FIG. 6A is a perspective view of an exemplary ground ring for a downholeelectrocrushing drill bit. FIG. 6B is a cross-sectional view of theground ring shown in FIG. 6A. Ground ring 650 provides a similarfunction and has similar features as ground ring 250 shown in FIG. 2 .

The shape of ground ring 650 may be selected to change the shape of theelectric field surrounding the electrocrushing drill bit duringelectrocrushing drilling. For example, the electric field surroundingthe electrocrushing drill bit may be designed so that the arc initiatesat an electrode and terminates on ground ring 650 or vice versa suchthat the arc initiates from ground ring 650 and terminates on theelectrode. The electric field changes based on the shape of the contoursof the edges of ground ring 650. For example, downhole edge 662 may havea sharp radius of curvature such that the electric field conditions atdownhole edge 662 are favorable for arc initiation and/or termination.Additionally, downhole edge 662 may be a distal portion of ground ring650 that engages with a portion of the wellbore, such as wellbore 116shown in FIG. 1 . Curve 665 on the inner perimeter of ground ring 650may have a gentle radius of curvature to such that the electric fieldconditions at curve 665 are not favorable for arc initiation and/ortermination. A radius of curvature of a transition is the radius of acircle of which the arc of the transition is a part. By way of example,a sharp radius of curvature may be a radius in the range ofapproximately 0.05 to approximately 0.15 inches, such as approximately0.094 inches, and a gentle radius of curvature may be a radius in therange of approximately 0.20 to approximately 1.0 inches or more, such asapproximately 1.0 inches or more, such as approximately 0.25 inches,approximately 0.5 inches, approximately 0.75 inches, or approximately1.0 inches. The gentle radius may be determined based on the geometry ofthe surrounding structures on electrocrushing drill bit 114 and theshape electric field for a given electrocrushing drilling operation. Forexample, the electric fields on electrode 208 may be a function of thegeometry of ground ring 250 and the geometry and material of insulator210. For example, the radius of the edge of electrode 208 and the shapeof electrode 208 may affect the interaction of electrocrushing drill bit114 with the rock. Additionally, the structure of ground ring 250 may beadjusted to change the electric field distribution on electrode 208.Further, the material used to form insulator 210 and the configurationof insulator 210 may be adjusted to change the electric field onelectrode 208. In some examples, the dielectric constant of theelectrocrushing drilling fluid and the geometry of the rock fragmentsand the wellbore during the drilling process may affect theinstantaneous electric field distribution on electrode 208. The featureson ground ring 650 having a sharp radius of curvature may have the sameor different sharp radius as features on the electrode having a sharpradius of curvature.

Ground ring 650 may include one or more fluid flow ports 660 on theouter perimeter of ground ring 650 to direct electrocrushing drillingfluid from around an electrode, out of the drilling field, and uphole toclear debris from the electrocrushing drilling field. The number andplacement of fluid flow ports 660 may be determined based on the flowrequirements of the electrocrushing drilling operation. For example, thenumber and/or size of fluid flow ports 660 may be increased to provide afaster fluid flow rate and/or larger fluid flow volume. Edge 668 of eachfluid flow port 660 may have a gentle radius of curvature such that theelectric field conditions at edge 668 of each fluid flow port 660 arenot favorable for arc initiation and/or termination.

Ground ring 650 may be manufactured from any material that can withstandthe conditions in the wellbore and support the downforce from the upholedrilling components, such as steel in the 41 family (often designated asthe 41xx family, for example 4140 steel), carbon alloyed steel,stainless steel, nickel and nickel alloys, copper and copper alloys,titanium and titanium alloys, chromium and chromium alloys, molybdenumand molybdenum alloys, doped ceramics, and combinations thereof. Asdescribed with respect to electrode 308, when an arc initiates orterminates at ground ring 650, the temperature at the initiation ortermination point increases such that the temperature melts the surfaceof ground ring 650. When the shock wave hits the melted surface ofground ring 650, a portion of the melted surface may separate from theremainder of ground ring 650 and be carried uphole with theelectrocrushing drilling fluid. Therefore, to prevent material loss, theareas of ground ring 650 having electric field conditions favorable toarc initiation and/or termination may be coated with or made from ametal matrix composite, as described in FIGS. 3A-3B.

Ground ring 650 may further include threads 670 along the inner diameterof ground ring 650. Threads 670 may engage with corresponding threads ona portion of an electrocrushing drill bit such that ground ring 650 isreplaceable during the electrocrushing drilling operation. Ground ring650 may be replaced if ground ring 650 is damaged by erosion or fatigueduring an electrocrushing drilling operation.

The thickness of wall 672 of ground ring 650 may be based on thediameter of ground ring 650 and/or the weight of the uphole componentsof the electrocrushing drilling system that are exerting downforce onground ring 650. For example, the thickness of wall 672 may range fromapproximately 0.25 inches to approximately 2 inches. The thickness ofwall 672 may be based on the diameter of ground ring 650 such that thethickness of wall 672 increases as the diameter of ground ring 650increases. Additionally, the thickness of wall 672 may taper such thatthe thickness is the smallest at downhole edge 662 and the largestbetween curve 664 and curve 665. For example, the thickness of wall 672may be approximately 0.3 inches at downhole edge 662 and increase toapproximately 0.8 inches between curve 664 and curve 665. The taperingof the thickness of wall 672 may provide annular clearance for the flowof electrocrushing drilling fluid to clear debris from between thebottom hole assembly to which the electrocrushing drill bit is attachedand the inner wall of the wellbore.

Diameter 674 of ground ring 650 may be based on the diameter of thewellbore and the annular clearance between the wellbore and the bottomhole assembly to which the electrocrushing drill bit is attached. Thediameter of the electrode contained within ground ring 650 on theelectrocrushing drill bit may be selected for drilling a particular typeof formation. For example, the diameter of the electrode may be selectedto optimize the electric field surrounding the electrocrushing drill bitand provide flow space for electrocrushing drilling fluid. Ground ring650 may have an outer diameter equal to the gauge of the wellbore to bedrilled by the electrocrushing drill bit or may have an outer diameterslightly smaller than the gauge of the wellbore to be drilled. Forexample, the outer diameter of ground ring 650 may be at least 0.03inches or at least 0.5 inches smaller than the gauge of the wellbore tobe drilled. In some examples, ground ring 650 may have features on theinner diameter of ground ring 650, such as curve 665, may have a gentleradius while features on the outer diameter of ground ring 650, such ascurve 664, may have a sharp radius such that the electrocrushing drillbit creates an overgauged wellbore during a drilling operation.

During the electrocrushing drilling operation, the electrode and groundring 650 may have opposite polarities to create electric fieldconditions such that arcs initiate at the electrode and terminate on theground ring or vice versa such that the arcs initiate at ground ring 650and terminate on the electrode. For example, the electrode may have apositive polarity while ground ring 650 has a negative polarity.

FIG. 7 is a perspective view of an electrocrushing drill bit includingmultiple electrodes and a ground ring. Electrocrushing drill bit 714 mayinclude multiple electrodes 708. Electrodes 708 may be similar toelectrode 208, shown in FIG. 2 and may have any of the features ofelectrodes 308, 408, and/or 508, shown in FIGS. 3A-5B, such as notches,dimples, serration, or other discontinuities. For example, whileelectrodes 708 are shown as rod-shaped in FIG. 7 , electrodes 708 may beconical shaped. Electrodes 708 may have different voltages applied toeach electrode 708 when electrical energy is applied to electrodes 708.For example, ground ring 750 and electrode 708 a may be at groundpotential and electrodes 708 b may have a peak voltage of 150 kV.

Electrocrushing drill bit 714 may additionally include solid insulator710 and ground ring 750. Solid insulator 710 may be similar to solidinsulator 210 shown in FIG. 2 . Ground ring 750 may be similar to groundring 250 shown in FIG. 2 and may have any of the features of ground ring650 shown in FIGS. 6A-6B.

The features of an electrocrushing drill bit described with respect toFIGS. 1-6B may be combined in any configuration. For example, FIG. 8 isa perspective view of an electrocrushing drill bit including multipleelectrodes arranged in multiple rows with an external ground ring and anintermediate ground ring. Electrocrushing drill bit 814 may includemultiple electrodes 808. Electrodes 808 may be similar to electrode 708,shown in FIG. 7 and may have any of the features of electrodes 308, 408,and/or 508, shown in FIGS. 3A-5B, such as notches, dimples, serration,or other discontinuities. For example, while electrodes 708 are shown asrod-shaped in FIG. 7 , electrodes 808 may be conical shaped. Electrodes808 may be shaped to facilitate fluid flow, including a tapered orairfoil shape. Electrodes 808 b may be arranged in a pattern of one ormore circular rows around center electrode 808 a Electrodes 808 may havedifferent voltages applied to different sets of electrodes when theelectrical pulse is applied to electrodes 808. For example, outer groundring 850 b, intermediate ground ring 850 a, and center electrode 808 amay be at ground potential and electrodes 808 b and 808 c may have apeak voltage of approximately 150 kV.

Electrocrushing drill bit 814 may additionally include ground rings 850a and 850 b. Ground ring 850 b may be similar to ground ring 250 shownin FIG. 2 and may have any of the features of ground ring 650 shown inFIGS. 6A-6B. Ground ring 850 a may have rectangular ports, circularports, or ports of other geometric shapes.

Electrocrushing drill bit 814 may be capable of electrically controlleddirectional drilling. A portion, for example approximately one-third, ofelectrodes 808 in FIG. 8 may be electrically connected and may fire at ahigher repetition rate than the other electrodes 808, for exampleapproximately two-thirds of electrodes 808. Electrocrushing drill bit814 may turn towards the slow repetition rate electrodes. In thismanner, electrocrushing drill bit 814 may be used to electrically steerthe drill during drilling operations by independently controlling therepetition rate of groups of electrodes 808.

FIG. 9 is a perspective view of an electrocrushing drill bit includingmultiple electrodes, an outer ground ring, and an intermediate groundring traversing the outer ground ring to divide the bit into threeregions. Electrocrushing drill bit 914 may include multiple electrodes908. Electrodes 908 are arranged in three groups within each of threesegments formed by the transverse ground ring. Electrodes 908 may besimilar to electrodes 808 or 708, shown in FIGS. 7 and 8 and may haveany of the features of electrodes 308, 408, and/or 508, shown in FIGS.3A-5B, such as notches, dimples, serration, or other discontinuities.For example, while electrodes 708 are shown as rod-shaped in FIG. 7 ,electrodes 908 may be conical shaped. Electrodes 908 may be shaped tofacilitate fluid flow, including a tapered or airfoil shape. Electrodes908 may have different voltages applied to different groups ofelectrodes when the electrical pulse is applied to electrodes 908. Forexample, outer ground ring 950 a and transverse ground structure 950 bmay be at ground potential and electrodes 908 may have a peak voltage ofapproximately 150 kV. While electrodes 908 are shown in FIG. 9 asarranged in three segments, electrodes 908 may be arranged in more orfewer segments.

Electrocrushing drill bit 914 may additionally include outer ground ring950 a and transverse ground structure 950 b. Ground ring 950 may besimilar to ground ring 250 shown in FIG. 2 and may have any of thefeatures of ground ring 650 shown in FIGS. 6A-6B. Outer ground ring 950a and transverse ground structure 950 b may have rectangular ports,circular ports, or ports of other geometric shapes.

Electrocrushing drill bit 914 may be capable of electrically controlleddirectional drilling. One group of electrodes 908 within one segmentformed by transverse ground structure 950 b may fire at a higherrepetition rate than the other groups of electrodes 908. Electrocrushingdrill bit 914 may turn towards electrodes 908 firing at a slowrepetition rate. In this manner, electrocrushing drill bit 914 may beused to electrically steer the drill during drilling operations byindependently controlling the repetition rate of groups of electrodes908.

FIG. 10 is a perspective view of an electrocrushing drill bit includingmultiple electrodes, an outer ground ring, and an intermediate groundring traversing the outer ground ring to divide the electrocrushingdrill bit into nine regions. Each of the nine regions enclosewedge-shaped electrode 1008. Electrocrushing drill bit 1014 may includemultiple electrodes 1008. Electrodes 1008 may be arranged into groups.For example, electrocrushing drill bit 1014 includes three groups ofthree electrodes 1008 each within each of nine segments formed bytransverse ground ring 1050. Each of electrodes 1008 may have the sameshape or may have different shapes as shown in FIG. 10 . In FIG. 10 ,electrodes 1008 are shown as wedge-shaped such that electrodes 1008 fitwithin the wedge-shaped segments formed by transverse ground structure1050 b. Alternatively, electrodes 1008 may be elliptical shaped or acombination of curved and straight lines to fit within the segmentsformed by transverse ground structure 1050 b. Electrodes 1008 may havedifferent voltages applied to different groups of electrodes atdifferent times to provide drilling function. For example, ground ring1050 a and transverse ground structure 1050 b may be at ground potentialand electrodes 1008 may have a peak voltage of approximately 150 kV.While FIG. 10 shows a multi-electrode configuration consisting of ninesegments and nine electrodes 1008, electrocrushing drill bit 1014 mayhave a configuration that consists of six electrodes, eight electrodes,twelve electrodes or some other number of electrodes 1008 according tothe parameters of the drilling operation.

Electrocrushing drill bit 1014 may additionally include transverseground structure 1050 b integral with or separate from outer ground ring1050 a. Outer ground ring 1050 a may be similar to ground ring 250 shownin FIG. 2 and may have any of the features of ground ring 650 shown inFIGS. 6A-6B. Outer ground ring 1050 a and transverse ground ring 1050 bmay have rectangular ports, circular ports, or ports of other geometricshapes between segments.

Electrocrushing drill bit 1014 may be capable of electrically controlleddirectional drilling. One group of electrodes 1008 within one group ofsegments formed by transverse ground structure 1050 b may fire at ahigher repetition rate than the other groups of electrodes 1008.Electrocrushing drill bit 1014 may turn towards electrodes 1008 firingat a slow repetition rate. In this manner, electrocrushing drill bit1014 may be used to electrically steer the drill during drillingoperations by independently controlling the repetition rate of groups ofelectrodes 1008.

FIG. 11 is a perspective view of an electrocrushing drill bit includingmultiple electrodes located within openings in a ground ring structure.Electrocrushing drill bit 1114 may include multiple electrodes 1108.Electrodes 1108 b may each be located within a port in ground ringstructure 1150. Each of electrodes 1108 may have the same shape, asshown in FIG. 11 , or may have different shapes. Electrodes 1108 may besimilar to electrodes 808 or 708, shown in FIGS. 7 and 8 and may haveany of the features of electrodes 308, 408, and/or 508, shown in FIGS.3A-5B, such as notches, dimples, serration, or other discontinuities.For example, while electrodes 1108 are shown as rod-shaped in FIG. 11 ,electrodes 1108 may be conical shaped. Electrodes 1108 may havedifferent voltages applied to different groups of electrodes atdifferent times to provide directional drilling function. For example,ground ring structure 1150 may be at ground potential and electrodes1108 may have a peak voltage of approximately 150 kV. While FIG. 11shows a multi-electrode configuration consisting of seven electrodes1108 within ground ring structure 1150, electrocrushing drill bit 1114may have a configuration that consists of four electrodes, tenelectrodes, or some other number of electrodes 1108 according to theparameters of the drilling operation.

Electrocrushing drill bit 1114 may additionally include ground ringstructure 1150 that may be flat and perpendicular to the direction oftravel of electrocrushing drill bit 1114. Ground ring structure 1150 mayalso include curved portions, as shown in FIG. 11 , to useelectrocrushing drill bit 1114 during directional drilling.

Electrocrushing drill bit 1114 may be capable of electrically controlleddirectional drilling. One or more electrodes 1108 may fire at a higherrepetition rate than the other electrodes 1108. Electrocrushing drillbit 1114 may turn towards electrodes 1108 firing at a slow repetitionrate. In this manner, electrocrushing drill bit 1114 may be used toelectrically steer the drill during drilling operations by independentlycontrolling the repetition rate of groups of electrodes 1108.

FIG. 12 is a perspective view of an electrocrushing drill bit includingmultiple electrodes arranged in rows, a central electrode, and a groundring. Electrocrushing drill bit 1214 may include multiple electrodes1208 b arranged in a row and central electrode 1208 a. Electrodes 1208may be similar to electrode 708, shown in FIG. 7 and may have any of thefeatures of electrodes 308, 408, and/or 508, shown in FIGS. 3A-5B, suchas notches, dimples, serration, or other discontinuities. For example,while electrodes 1208 are shown as rod-shaped in FIG. 12 , electrodes1208 may be conical shaped. Electrodes 1208 may be shaped to facilitatefluid flow, including a tapered or airfoil shape. Electrodes 1208 mayhave different voltages applied to different sets of electrodes 1208.For example, outer ground ring 1250, and center electrode 1208 a may beat ground potential and electrodes 1208 b may have a peak voltage ofapproximately 150 kV.

Electrocrushing drill bit 1214 may additionally include ground ring1250. Ground ring 1250 may be similar to ground ring 250 shown in FIG. 2and may have any of the features of ground ring 650 shown in FIGS.6A-6B. Ground ring 1250 may have one or more projection 1252 built intothe ground ring 1250 as shown in FIG. 12 . Projections 1252 might becylindrical, as shown in FIG. 12 , or square shaped, or triangular, orany other suitable shape that provides control of the drilling rate.

Electrocrushing drill bit 1214 may be capable of electrically controlleddirectional drilling. One or more electrodes 1208 in FIG. 12 may beelectrically connected and may fire at a higher repetition rate than theother electrodes 1208. Electrocrushing drill bit 1214 may turn towardselectrodes 1208 firing at a slow repetition rate. In this manner,electrocrushing drill bit 1214 may be used to electrically steer thedrill during drilling operations by independently controlling therepetition rate of groups of electrodes 1208.

FIG. 13 is a flow chart of exemplary method for drilling a wellbore.Method 1300 may begin and at step 1310 a drill bit may be placeddownhole in a wellbore. For example, drill bit 114 may be placeddownhole in wellbore 116 as shown in FIG. 1 .

At step 1320, electrocrushing drilling fluid may be provided to thedownhole drilling field through a fluid flow opening in the center ofthe electrode, along with fluid flow over the top of the electrode. Forexample, as described above with reference to FIG. 3 , an electrode mayinclude a fluid flow opening in approximately the center of theelectrode. Electrocrushing drilling fluid may flow from the drill stingout of the fluid flow opening and into the drilling area. Once in thedrilling area, the flow of the electrocrushing drilling fluid may bedirected by one or more slots on the face of the electrode.

At step 1330, electrical energy may be provided to an electrode and aground ring of the drill bit. For example, as described above withreference to FIGS. 1 and 2 , a pulse-generating circuit may beimplemented within pulsed-power tool 230 of FIG. 2 . And as describedabove with reference to FIG. 2 , pulsed-power tool 230 may receiveelectrical power from a power source on the surface, from a power sourcelocated downhole, or from a combination of a power source on the surfaceand a power source located downhole. The electrical power may beprovided to the pulse-generating circuit within pulse-power tool 230.The pulse generating circuit may be coupled to an electrode (such aselectrode 208 shown in FIG. 2 ) and a ground ring (such as ground ring250 or 650 shown in FIGS. 2 and 6 , respectively) of drill bit 114.

At step 1340, an electrical arc may be formed between the firstelectrode and the second electrode of the drill bit. Thepulse-generating circuit may be utilized to repeatedly apply a highelectric potential, for example up to or exceeding approximately 150 kV,across the electrode. Each application of electric potential may bereferred to as a pulse. When the electric potential across the electrodeand ground ring is increased enough during a pulse to generate asufficiently high electric field, an electrical arc forms through a rockformation at the bottom of the wellbore. The arc may initiate at aportion of the electrode having a sharp radius of curvature andterminate on a portion of the ground ring having a sharp radius ofcurvature, or vice versa such that the arc initiates on a portion of theground ring having a sharp radius of curvature and terminate on aportion of the electrode having a sharp radius of curvature. The arctemporarily forms an electrical coupling between the electrode and theground ring, allowing electric current to flow through the arc inside aportion of the rock formation at the bottom of the wellbore.

At step 1350, the rock formation at an end of the wellbore may befractured by the electrical arc. For example, as described above withreference to FIGS. 1 and 2 , the arc greatly increases the temperatureof the portion of the rock formation through which the arc flows as wellas the surrounding formation and materials. The temperature issufficiently high to vaporize any water or other fluids that may betouching or near the arc and may also vaporize part of the rockformation itself. The vaporization process creates a high-pressure gaswhich expands and, in turn, fractures the surrounding rock.

At step 1360, fractured rock may be removed from the end of thewellbore. For example, as described above with reference to FIG. 1 ,electrocrushing drilling fluid 122 may move the fractured rock away fromthe electrode and uphole away from the bottom of wellbore 116. The stepsof method 1300 may be repeated until the wellbore has been drilled orthe drill bit needs to be replaced. Subsequently, method 1300 may end.

Modifications, additions, or omissions may be made to method 1300without departing from the scope of the disclosure. For example, theorder of the steps may be performed in a different manner than thatdescribed and some steps may be performed at the same time.Additionally, each individual step may include additional steps withoutdeparting from the scope of the present disclosure.

Embodiments herein may include:

A electrocrushing drill bit including a bit body; an electrode coupledto a power source and the bit body, the electrode having a distalportion for engaging with a surface of a wellbore; a ground ring coupledto the bit body proximate to the electrode and having a distal portionfor engaging with the surface of the wellbore, the electrode and theground ring positioned in relation to each other such that an electricfield produced by a voltage applied between the ground ring and theelectrode is enhanced at a portion of the electrode proximate to thedistal portion of the electrode and at a portion of the ground ringproximate to the distal portion of the ground ring; and an insulatorcoupled to the bit body between the electrode and the ground ring.

A downhole drilling system including a drill string; a power source; anda drill bit coupled to the drill string and the power source. The drillbit includes a bit body; an electrode coupled to a power source and thebit body, the electrode having a distal portion for engaging with asurface of the wellbore; a ground ring coupled to the bit body proximateto the electrode and having a distal portion for engaging with thesurface of the wellbore, the electrode and the ground ring positioned inrelation to each other such that an electric field produced by a voltageapplied between the ground ring and the electrode is enhanced at aportion of the electrode proximate to the distal portion of theelectrode and at a portion of the ground ring proximate to the distalportion of the ground ring; and an insulator coupled to the bit bodybetween the electrode and the ground ring.

A method including placing a drill bit downhole in a wellbore;supporting the weight of the drill bit and a drill string with a drillstring support; providing electrical energy to the drill bit; providingelectrocrushing drilling fluid to the drill bit; forming an electricalarc between the portion of the electrode proximate to the distal portionof the electrode and the portion of the ground ring proximate to thedistal portion of the ground ring of the drill bit; fracturing a rockformation at an end of the wellbore with the electrical arc; andremoving fractured rock from the end of the wellbore with theelectrocrushing drilling fluid. The drill bit includes a bit body; anelectrode coupled to a power source and the bit body, the electrodehaving a distal portion for engaging with a surface of a wellbore; aground ring coupled to the bit body proximate to the electrode andhaving a distal portion for engaging with the surface of the wellbore,the electrode and the ground ring positioned in relation to each othersuch that an electric field produced by a voltage applied between theground ring and the electrode is enhanced at a portion of the electrodeproximate to the distal portion of the electrode and at a portion of theground ring proximate to the distal portion of the ground ring; and aninsulator coupled to the bit body between the electrode and the groundring.

Each of embodiments A, B, and C may have one or more of the followingadditional elements in any combination: Element 1: wherein the electrodefurther includes a stem adjacent to the body and an opening extendingthrough the stem and the body to the face of the electrode. Element 2:wherein the electrode further includes a slot in the face of theelectrode. Element 3: wherein the slot is a channel in the face of theelectrode. Element 4: wherein the slot extends through the body of theelectrode. Element 5: wherein the edge of the face of the electrodeincludes a notch. Element 6: wherein the electrode further includes astem adjacent to the body and a spring extending through a center of thestem to the body of the electrode. Element 7: wherein the electrodefurther includes a stem; and a transition between the body and the stemof the electrode has a gentle radius of curvature. Element 8: whereinthe ground ring further includes a fluid flow port. Element 9: whereinan edge of the fluid flow port on the ground ring has a gentle radius ofcurvature. Element 10: wherein the electrode further includes a stem;and the electrocrushing drilling fluid is provided to the drill bit viaa fluid flow opening extending through the stem to the face of thegenerally conical shaped electrode. Element 11: wherein a flow of theelectrocrushing drilling fluid is modified by a slot in a face of theelectrode. Element 12: wherein the electric arc initiates on the distalportion of the electrode and terminates on the distal portion of theground ring. Element 13: wherein the electric arc initiates on thedistal portion of the ground ring and terminates on the distal portionof the electrode. Element 14: further comprising maintaining contactbetween the face of the electrode and the rock formation by compressinga spring extending through a center of a stem adjacent to the body ofthe electrode. Element 15: wherein an edge of the electrode has a firstsharp radius of curvature and the distal portion of the ground ring hasa second sharp radius of curvature, the first sharp radius of curvatureand the second sharp radius of curvature have a radius of betweenapproximately 0.05 inches and approximately 0.15 inches. Element 16:further comprising a drill string support coupled to the bit body.Element 17: wherein the ground ring is the drill string support. Element18: wherein the ground ring includes a projection extending from theground ring. Element 19: wherein the ground ring includes an outerground ring and a transverse ground structure. Element 20: wherein theground ring includes multiple ground rings. Element 21: wherein theelectrode includes a plurality of electrodes. Element 22: wherein theplurality of electrodes are arranged in a circular pattern on the bitbody. Element 23: wherein the electrode has a shape selected from thegroup consisting of conical, cylindrical, rod, triangular, elliptical,wedge, taper, and airfoil. Element 24: wherein providing electricalenergy to the drill bit includes providing electrical energy to a subsetof the plurality of electrodes at a higher repetition rate than anothersubset of the plurality of electrodes. Element 25: wherein the electrodefurther includes a stem adjacent to the body and a piston positionedwithin a center of the stem to the body of the electrode.

Although the present disclosure has been described with severalembodiments, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosureencompasses such various changes and modifications as falling within thescope of the appended claims.

What is claimed is:
 1. An electrocrushing drill bit, comprising: a bitbody; an electrode coupled to a power source and the bit body, theelectrode having a distal portion for engaging with a surface of awellbore; a ground ring coupled to the bit body proximate to theelectrode and having a distal portion for engaging with the surface ofthe wellbore, the electrode and the ground ring positioned in relationto each other such that an electric field produced by a voltage appliedbetween the ground ring and the electrode is enhanced at a portion ofthe electrode proximate to the distal portion of the electrode and at aportion of the ground ring proximate to the distal portion of the groundring. wherein the ground ring further includes a fluid flow port; and aninsulator coupled to the bit body between the electrode and the groundring.
 2. The electrocrushing drill bit of claim 1, wherein the electrodefurther includes a stem adjacent to a body of the electrode and anopening extending through the stem and the body of the electrode to theface of the electrode.
 3. The electrocrushing drill bit of claim 1,wherein the electrode further includes a stem adjacent to the body and aspring extending through a center of the stem to the body of theelectrode.
 4. The electrocrushing drill bit of claim 1, wherein: theelectrode further includes a stem; and a transition between a body ofthe electrode and the stem of the electrode has a radius of curvaturebetween 0.15-inches and 1.0-inches.
 5. The electrocrushing drill bit ofclaim 1, wherein an edge of the electrode has a first sharp radius ofcurvature and the distal portion of the ground ring has a second sharpradius of curvature, the first sharp radius of curvature and the secondsharp radius of curvature have a radius of between approximately 0.05inches and approximately 0.15 inches.
 6. The electrocrushing drill bitof claim 1, wherein the ground ring is a drill string support.
 7. Theelectrocrushing drill bit of claim 1, wherein the electrode has a shapeselected from the group consisting of conical, cylindrical, rod,triangular, elliptical, wedge, taper, and airfoil.
 8. Theelectrocrushing drill bit of claim 1, wherein the distal portion has anedge with a radius of curvature between 0.05-inches to 0.15-inches. 9.The electrocrushing drill bit of claim 1, wherein an edge of the slothas a radius of curvature between 0.05-inches and 0.15-inches.
 10. Theelectrocrushing drill bit of claim 1, wherein the electrode includes aslot.
 11. The electrocrushing drill bit of claim 2, wherein the slotextends through the body of the electrode.
 12. The electrocrushing drillbit of claim 1, wherein an edge of the fluid flow port on the groundring has a radius of curvature.
 13. The electrocrushing drill bit ofclaim 2, wherein the edge of the fluid flow port on the ground ring hasa sharp radius of curvature.
 14. The electrocrushing drill bit of claim2, wherein the edge of the fluid flow port on the ground ring has agentle radius of curvature.
 15. A downhole drilling system comprising: adrill string; a power source; and a drill bit coupled to the drillstring and the power source, the drill bit including: a bit body; anelectrode coupled to a power source and the bit body, the electrodehaving a distal portion for engaging with a surface of a wellbore; aground ring coupled to the bit body proximate to the electrode andhaving a distal portion for engaging with the surface of the wellbore,the electrode and the ground ring positioned in relation to each othersuch that an electric field produced by a voltage applied between theground ring and the electrode is enhanced at a portion of the electrodeproximate to the distal portion of the electrode and at a portion of theground ring proximate to the distal portion of the ground ring. whereinthe ground ring further includes a fluid flow port; and an insulatorcoupled to the bit body between the electrode and the ground ring. 16.The downhole drilling system of claim 15, wherein the electrode furtherincludes a stem adjacent to a body of the electrode and an openingextending through the stem and the body of the electrode to the face ofthe electrode.
 17. The downhole drilling system of claim 15, wherein theelectrode further includes a stem adjacent to the body and a springextending through a center of the stem to the body of the electrode. 18.A method comprising: placing a drill bit downhole in a wellbore, thedrill bit including: a bit body; an electrode coupled to a power sourceand the bit body, the electrode having a distal portion for engagingwith a surface of a wellbore; a ground ring coupled to the bit bodyproximate to the electrode and having a distal portion for engaging withthe surface of the wellbore, the electrode and the ground ringpositioned in relation to each other such that an electric fieldproduced by a voltage applied between the ground ring and the electrodeis enhanced at a portion of the electrode proximate to the distalportion of the electrode and at a portion of the ground ring proximateto the distal portion of the ground ring. wherein the ground ringfurther includes a fluid flow port; and an insulator coupled to the bitbody between the electrode and the ground ring. supporting the weight ofthe drill bit and a drill string with a drill string support; providingelectrical energy to the drill bit; providing electrocrushing drillingfluid to the drill bit; forming an electrical arc between the portion ofthe electrode proximate to the distal portion of the electrode and theportion of the ground ring proximate to the distal portion of the groundring of the drill bit; fracturing a rock formation at an end of thewellbore with the electrical arc; and removing fractured rock from theend of the wellbore with the electrocrushing drilling fluid.
 19. Themethod of claim 18, wherein the electrode further includes a stemadjacent to a body of the electrode and an opening extending through thestem and the body of the electrode to the face of the electrode.
 20. Themethod of claim 18, wherein the electrode further includes a stemadjacent to the body and a spring extending through a center of the stemto the body of the electrode.